Comparison of halide receptors based on H, halogen, chalcogen, pnicogen, and tetrel bonds.

A series of halide receptors are constructed and the geometries and energetics of their binding to F-, Cl-, and Br- assessed by quantum calculations. The dicationic receptors are based on a pair of imidazolium units, connected via a benzene spacer. The imidazoliums each donate a proton to a halide in a pair of H-bonds. Replacement of the two bonding protons by Br leads to binding via a pair of halogen bonds. Likewise, chalcogen, pnicogen, and tetrel bonds occur when the protons are replaced, respectively, by Se, As, and Ge. Regardless of the binding group considered, F- is bound much more strongly than are Cl- and Br-. With respect to the latter two halides, the binding energy is not very sensitive to the nature of the binding atom, whether H or some other atom. But there is a great deal of differentiation with respect to F-, where the order varies as tetrel > H ∼ pnicogen > halogen > chalcogen. The replacement of the various binding atoms by their analogues in the next row of the periodic table enhances the fluoride binding energy by 22-56%. The strongest fluoride binding agents utilize the tetrel bonds of the Sn atom, whereas it is I-halogen bonds that are preferred for Cl- and Br-. After incorporation of thermal and entropic effects, the halogen, chalcogen, and pnicogen bonding receptors do not represent much of an improvement over H-bonds with regard to this selectivity for F-, even I which binds quite strongly. In stark contrast, the tetrel-bonding derivatives, both Ge and Sn, show by far the greatest selectivity for F- over the other halides, as much as 1013, an enhancement of six orders of magnitude when compared to the H-bonding receptor.